Metadata education suggestions and materials for:
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Data Models |
Learning Material | Preparatory topics | Complementary topics | Vocabulary
Motivation
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Knowledge
General:
The real world is too complex for our immediate and direct understanding, so we create "models" or abstractions of reality that are intended to have some similarity with selected aspects of the real world. A spatial database is a collection of spatially referenced data that acts as a model of reality.
There are two main types of data models:
Objects: objects with discrete boundaries represented by geometric features
Fields: continuous phenomena such as elevation, temperature and soil chemistry; they exist everywhere (every point has an elevation or temperature); they are not discrete entities.
Raster: a grid of cells or pixels, usually square, but may also be rectangular or even hexagonal. Other variations of the raster model include vertical arrays and quadtree structure
Vector: objects are represented as geometric entities, including points, "spaghetti" lines, arc/node lines, networked lines with directionality, whole polygons, and topological polygons. Continuous fields may also be represented by geometric entities such as triangulated irregular networks
Object-oriented: real world objects are used as the basis for abstraction, instead of geometric objects such as squares, lines, and polygons.
Understanding the type of data model a particular dataset is created and stored as is important, since the type of functions or transformations that can be performed on the data will be dependant on how it is represented and organized. The logical data model not only describes how the data is represented, but it should also describe how entities or features are related to one another (such as topological relationships or object-pair relationships).
The FGDC's Content Standard for Digital Geospatial Metadata (CDGSM) and the ISO metadata standard for geographic information have sections for reporting the way data is represented and organized in its logical data model. The Spatial Organization Information section in the CSDGM and the Spatial Data Representation section in the ISO standard both provide means for describing the data as either a raster model or a vector model. Under these distinctions, further metadata elements are provided for describing the number and type of raster or geometric elements, and for vector data there are also methods for describing the level of topological relationship.
See the topic on how to read the Spatial Organization section for more details.
Neither metadata content standard provides structured elements for describing all variations of logical data models (such as triangulated irregular networks) or relationships (special types of segmentation, which define unusual "intersections" such as highway overpasses). Metadata also does not provide elements for specifically describing conceptual models (such as object-oriented data). Other sections of the metadata document, including the Identification Information section (Abstract and Native Environment) and the Entity and Attribute Information section, may contain additional information about the data's structure. Finally, the Process Steps of the Data Quality section may contain valuable information about how the data was created (abstracted) or transformed (conversion between different data models).
Advanced material:
Cross-Media Database Normalization Across Various Metadata Standards
Lesch, J. 2001. Proceedings of the Twenty-First Annual ESRI User Conference.
View paper
This paper describes new methods of organizing multi-media digital data in a unified multirelational database design. Particulur focus is given to the development of the cross media data model including normalization across various metadata standards.
Goodchild, M 1997. Representing Fields. NCGIA Core Curriculum in GIScience, http://www.ncgia.ucsb.edu/giscc/units/u054/u054.html, posted October 23, 1998.
NCGIA 1990 Core Curriculum, Unit 10: Spatial databases as models of reality
NCGIA 1990 Core Curriculum, Unit 11: Spatial objects and database models
NCGIA 1990 Core Curriculum, Unit 12: Relationships among spatial objects